Fast sequential switch with adjustable delay

ABSTRACT

A fast sequential switch utilizes a relatively long sample of ptype material, such as indium antimonide, which is capable of impact ionization activated by injected electrons. A high average electric field is created in the device by an external pulsed voltage source. Electron injection may be from the source, by the photoelectric effect, or the like. The injected electrons produce an impact ionization wavefront which travels the length of the device from cathode to anode in a very short period of time, on the order of 1.4 nanoseconds for a device 1.83 mm long. The wavefront is accompanied by a drastic decrease in the localized electric field and resistance behind the wavefront. A plurality of side contacts or probes located along the length of the device are connected to a corresponding plurality of detectors which may monitor either the device&#39;&#39;s potential or resistance. When the impact ionization wavefront passes the location of a particular contact or probe, the detector is switched on and maintained on until the biasing pulse terminates. Probes displaced by approximately 1 mm are activated approximately 0.3 nanoseconds apart.

1 Ancker-Johnson et al.

[ 1 FAST SEQUENTIAL SWITCH WITH ADJUSTABLE DELAY [75] Inventors: Betsy Ancker-Johnson, Seattle,

Wash; Charles L. Dick, Jr., New

York, NY.

[73] Assignee: The Boeing Company, Seattle,

' Wash.

[22] Filed: Sept. 21, 1972 [21] Appl. No.: 290,836

[521 US. Cl........ 357 /13, 357/3. 357/57. 357m [51] Int. Cl. H011 13/00 [58] Field of Search 317/234, 235

[56] References Cited UNITED STATES PATENTS 3,396,283 8/1968 Glicksman et a1. 307/309 3,434,008 3/1969 Sandbank 315/169 3,496,024 2/1970 Ruehrwe in 136/89 3,585,609 6/1971 Robrock 340/173 3,716,424 2/1973 Schoolar 148/175 Primary Examiner-Rudolph V. Rolinec Assistant Examiner-E. Wojciechowicz Attorney, Agent, or Firm-Christensen, OConnor, Garrison & Havelka [451 June 25, 1974 [5 7] ABSTRACT A fast sequential switch utilizes a relatively long sample of p-type material, such as indium antimonide, which is capable of impact ionization activated by injected electrons. A high average electric field is created in the device by an external pulsed voltage source. Electron injection may be from the source, by the photoelectric effect, or the like. The injected electrons produce an impact ionization wavefront which travels the length of the device from cathode to anode in a very short period of time, on the order of 1.4 nanoseconds for a device 1.83 mm long. The wavefront is accompanied by a drastic decrease in the 10- calized electric field and resistance behind the wave- 7 front. A plurality of side contacts or probes located along the length of the device are connected to a corresponding plurality of detectors which may monitor either the devices potential or resistance. When the impact ionization wavefront passes the locationof a 8 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION This invention relates to switches using semiconductor materials and more particularly, to such a semiconductor switch capable of providing very fast sequential rial which provides reliable operation with a variable and reproducible time delay between each switching action in the sequence.

' It is a further object of this invention to provide such a sequential switching device which is very fast in its operation, providing-switching actions insubnanosecond times.

SUMMARY OF THE INVENTION These objects and others are achieved, briefly, pulsing a relatively long block of p-type semiconductor material which is capable of impact ionizationwhen maintained within a predetermined range of temperatures with a voltage pulse sufficient to create an impact ionization wavefront emanating from the cathode end of the block, and by providing a plurality of detectors coupled to a corresponding plurality of electrodes spaced along the length of the block from the cathode to the anode which are sequentially switched on in response to said impact ionization wavefront.

BRIEF DESCRIPTION OF THE DRAWINGS The invention can perhaps best be understood by reference' to the'following portion of the specification,

taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating the fast sequential switch of this invention;

FIG. 2 illustrates the variation in excess carrier conductivity along the block of p-type material, for three different applied. electric fields and for successive times after the applicationof the fields;

FIG. 3 illustrates the variation in electric field strength along the blocks length for two different applied electric fields and for successive times after the application of the fields; and

FIG. 4 is a graph. illustrating the relation between wavefront velocity of an injection wavefront or impact ionization wavefront along the block and the magnitude of average electric applied field.

DESCRIPTION OF A PREFERRED EMBODIMENT v The present invention achieves its fast detection times and high gain by utilizing the phenomenon of impact ionization. The impact ionization phenomenon has been recorded and verified in several types of semiconductor materials. The physical conditions for initiation of impact ionization in a semiconductor depend on telluride whether the semiconductor is p-type material or n-type material. In either case, it is necessary that electrons be removed from their valenceband and be elevated to a conduction band. This elevation occurs through electron collisions with already free" conduction electrons which transfer the requisite energy to the valence electrons. Generally. under the condition producing impact ionization, the initiating electrons have sufficient density and energy to knock electrons out of the valence band into the conduction band and to transmit to the newly produced conduction electrons sufficient energy so that they may in turn engage in additional ionizing collisions. stated another way, it must be statistically probable that the initiating conduction electrons produce a chain-reaction in which the number of conduction electrons multiplies rapidly.

In order for this process to occur, the material must be maintained within a range of rather low temperatures and must be subjected to an electric field of minimum magnitude, so that sufficient energy is imparted to the conduction electrons. It is preferred that the energy gap between the valence and conduction bands be relatively small so as to limit the magnitude of biasing voltage required. In addition, there must be a sufficient number of free or conducting charge carriers in the semiconductor material. In certain p-type materials,

such as indium antimonide (InSb) mercury cadmium (Hg Cd Te). and lead tin telluride (Pb ,Sn Te), it has been estimated that neither the thermal equilibrium electrons nor the holes initiate impact ionization, presumably because the thermal equilibrium electrons are too few in number to start an avalanche and the holes are too massive to acquire the requisite energy forionizing collisions in the times observed for impact ionization with given applied fields. Rather, it has been shown that injected electrons are required to initiate impact ionization in p-type InSb.

When p-type lnSb is subjected to an externallyapplied electric field having a strength from a few volts/cm to approximately 500 volts/cm, both electrons and holes. are injected from the source in large quantities and in approximately equal-densities. lt'is known that the excess electrons which are injected at the cathode of the material propagate to the anode behind a well-defined leading edge, or wavefront, with velocity dependent on electric field strength. After the wavefront reaches the anode, the excess carrier conductivity in the material, 0;,(x) and therefore current increases exponentially'with time to a non-equilibrium steady state condition. The average velocity for this wavefront is on the order of 5.6 X 10 cm/sec. Within the material, the localized electric field strength E(x) decreases behind the front and increases ahead of it.

For impact ionization to occur in p-type lnSb, the electric field strength E(x) at any point within the material must exceeda threshold field strength E The lowest measured value of E for injection from a voltage source has been 490 volts/cm. When electrons are injected and accompanied by average fields that greatly exceed E e.g. is E= 1,200 volts/cm. they initiate an avalanche process'once they penetrate the material a very short distance, too short to measure with existing techniques, but probably approximately 10 pm. After the injected electrons produce impact ionization in a thin layer at the cathode of the material, the field E(x) there drops and increases elsewhere in the material, as the average applied field E is constant. The newly produced conduction electrons at the front of this thin layer then acquire drift velocities even greater than the injected electrons had and cause impact ionization in an adjacent layer of the material. Thus, an impact ionization wavefront, initiated adjacent to the cathode, propagates along the length of the device to the anode.

If the electron injection is accompanied by average fields E less than that required for producing ii immediately after injection, e.g. less than 1,200 volts/cm, the

injected electrons will penetrate some distance into the material before impact ionization is initiated when the localized field E(.t) exceeds E,-,,,. If the average field E is below a certain value, approximately 300 volts/cm, the localized field strength E(.\') can never exceed E and a wavefront composed of injected electrons only traverses the device.

The phenomena of injection and impact ionization can be better understood by considering the graphs in FIGS. 2 and 3. In FIG. 2, a sample 10 of InSb is shown having a cathode at x O and an anode at x =a. In one embodiment, a 1.83 mm. Curves (A) of FIG. 2 illustrate the excess carrier conductivity a,.,.(x) along the device for a period of time from 2 to 50 nanoseconds (ns) after the application of an average electric field E 170 volts/cm at time t= 0. In this case, E(x) is everywhere less than. E,-,,,,, and hence conduction is a result of only electron injection at the cathode. The passage of the injection wavefront from the cathode to the anode is illustrated by the increase in cr ,,(x) with time. The time required for the wavefront to reach the anode after application of the electric field is approximately 32 nanoseconds, in this example.

In Curves (B) of FIG. 2, the variation in 0,,(x) is shown for a time period from 2 to l 2 nanoseconds after the application of an average field E 420 volts/cm at time 0. Since E is less than Eii Conduction is initially by injection.- Thus, at 6 nanoseconds, the injection wavefront has travelled approximately 0.8 mm from the cathode at x 0. However, at 8 nanoseconds, impact ionization has occurred and the impact ionization wavefront, signified by auniformconductivity T U), has travelled at a very high rate of speed toward the anode x a. The required time for the injected and impact ionization waves to travel to the anode was measured to be 8.4 nanoseconds after the'application of the external electric field. Impact ionization occurs at approximately 0.9 mm from the cathode, in this example.

As previously explained, the localized field E(x) increases ahead of an injection wavefront. With reference now to Curves (A) in F IG. 3, which show E(x) for the applied field E 420 volts/cm of Curves (B) in FIG. 2, it can be seen that at 6 nanoseconds, the localized field E(x) is at no place greater than Em that is, 490 volts/cm, in this case. However, the field E(x) is increasing with time and at 8 nanoseconds, the field E(x) has exceeded 490 volts/cm at 0.9 mm and impact ionization is initiated.

Once the impact ionization wavefront is formed, it travels so fast through the remainder of the device 10 that, with increasing time, a uniformly distributed, growing conductivity a,. (x) is observed in the anode half again with reference to Curves (B) in FIG. 3.

Curves (Q) of FIG. 2 show 03,,(x) for an external applied field E 7 l volts/cm at time t= O. In this case, very little injection occurs before impact ionization is initiated. By comparison with Curves (B) of FIG. 3, which show the corresponding variation in localized electric field strength E(x), it can be seen that E(x) exceeds E41,; at approximately 1.0 nanoseconds, at which time the injection wavefront is propagated approximately 0.5 mm from the cathode. The measured transit time in this case is 1.4 nanoseconds. Again, a uniformly distributed, growing conductivity is observed. From these measurements, the velocity of the impact ionization wavefront in p-type lnSb at 77 K has been calculated to equal'3.2 X 10" cm/sec.

Once the impact ionization wavefront is initiated, the conductivity 0',.,(.i') increases exponentially to a steadystate value and remains high until the external electric field is removed.

Now turning to FIG. 1,'a long block 10 of p-type lnSb capable of impact ionization forms the basic of the device. The block 10 has a cathode region to which a first electrical contact 12 is attached, and an anode region to which a second electrical contact 14 is attached. A load resistor R is connected from contact 14 to ground, and a pulsed voltage source 16 providing an output pulse V, is connected from electrical contact 12 to ground.

The block 10 is maintained in a controlled temperature environment 20 by means, not illustrated, which are conventional in the art. The temperature of environment 20 is dependent on the range at which the ptype material comprising block 10 has the capability of providing significant impact ionization. For p-type InSb, 80 K is preferred.

Pulsed voltage source 16 provides an output voltage pulse of magnitude V with respect to ground potential. V must be chosen so that the localized field strength E(x) at some point within the block 10 exceeds E upon electron injection from the source 16 to allow im pact ionization to occur. Preferably, V, is high enough to cause an impact ionization wavefront to be initiated in the cathode region adjacent electrical contact 12.

Arranged along the length of the block 10 are a plurality of contacts 18A, 18B, 18C, 18D, 18E and 18F. Contacts 18A and 18B are located on opposite sides of the block 10 at the same distance x from the cathode end of the block 10 and are connected in turn to a first detector 22. Likewise, contacts 18C and 18D are located at a second distance x, from the cathode end of the block 10 and are connected in turn to a second de-.

tector 24. Contact 18E is located at a further distance x from the cathode end of block 10 and is connected to adetector 26, and contact 18F is located at distance x,, and connected to a detector 28. All detectors are referenced to ground potential.

Detectors 22 and 24 are responsive to a resistance between their respective electrodes 18A, 18B and 18C, 18D. Detectors 26 and 28, on the other hand, are voltage detectors which are responsive to the potential .difference between the potential on their responsive contacts 18E, 18F, and ground.

It will be remembered that as the impact ionization wavefront travels the length of the block 10, the conductivity 0,,(x) increases drastically behind the wavefront and the localized electric field strength E(x) drops. Accordingly, as the wavefront passes x and x,, the resistance between the respective electrodes 18A,

18B and 18C, 18D drops drastically and detectors 22 tween the contacts 18E, 18F and ground also drops drastically and detectors 26 and 28 are'actuated.

It will be recognized by those skilled in the art that many types or combinations of resistance or voltage detectors be used, it only being required that each detector have a fast enough response time to make the switching action caused by the impact ionization wavefront meaningful. A preferred embodiment of each detector would, according to the present state of the art, include a fast avalanche transistor for switching.

That the sequential switch provides fast and reproducible operations can be visualized from a consideration of FIG. 4. The curve illustrates two saturation levels for wavefront velocity v in p-type lnSb at 77 K. The first level is approximately cm/sec. and represents the maximum drift velocity that injected electrons can attain. As expected from previous discussi on, injection predominates at average electric fields E below 400 volts/cm. At higher average electric fields, the conduction becomes a mixture of injection and impact ionization, as typified by Curves (B) of FIG. 2, and the velocity v increases. The higher or second saturation level is approximately 3.2 X 10 cm/sec. and represents the impact ionization velocity. The second saturation level is reached at an average electric field E of 1,200 volts/cm. It can be seen that the velocity v where impact ionization is the sole method of conduction is apsequence and with the same delays betweenswitching actions for every operation To change the delay between switching actuations, only the relative position of the contacts associated with any detector need be shifted along the block 10 with respect to the cathode at .r 0.

lf impact ionization is initiated adjacent to the cathode in a 2 mm long block, it can be shown that the impact ionization wave traverses the length of the diode from cathode to anode in less than 1 nanosecond. The switching actions are therefore provided in the subnanosecond range.

The voltage source 16 preferably comprises a square wave generator or other pulsed source, as continuous biasing would result in excess heating of the block 10 at the field strengths indicated. Also, a pulsed source 6 allows synchronized operation. That is, as impact ionization is terminated when an electric field is removed from the device, the termination of each biasing pulse resets the device for a subsequent cycle of operation.

What is claimed is:

1. An apparatus for providing a fast sequential switching operation in the subnanosecond range, com prising:

a. a block primarily composed of p-type semiconductor material having a cathode end and an anode end, said p-type material being capable of impact ionization in response to electron injection when maintained within a predetermined range of temperatures,

b. means maintaining said block at one of said temperatures in said predetermined range,

c. pulse generator means connected to said cathode end and said anode end for biasing said block with a voltage pulse having a magnitude sufficient to allow creation of an impact ionization wavefront upon electron injection, said impact ionization wavefront thereafter traveling from said cathode to said anode at a very high rate of speed,

d. a plurality of electrodes spaced along the length of said block intermediate said cathode and said anode ends, and

e. a plurality of detectors, each of said plurality of detectors being coupled to at least one of said plurality of electrodes and being adapted to be actuated upon passage of said impact ionization wavefront by its respective electrodes.

2. An apparatus as recited in claim 1, wherein at least one of said detectors is responsive to resistance ofsaid block between its corresponding electrodes.

3. An apparatus as recited in claim 1, wherein at least one of said detectors is responsive to the potential of said block at its corresponding electrode.

4. An apparatus as recited in claim 1, wherein said p-type material is indium antimonide.

5. An apparatus as recited in claim 4, wherein the magnitude and polarity of said biasing voltage pulse is sufficient to create an impact ionization wavefront adjacent said cathode end.

6. An apparatus as recited in claim 1, wherein the magnitude and polarity of said biasing voltage pulse is sufficient to create an impact ionization wavefront adjacent said cathode end.

7. An apparatus as recited in claim 1, wherein said p-type material comprises mercury cadmium telluride.

8. An apparatus as recited in claim 1, wherein said p-type material comprises lead tin telluride. l 

1. An apparatus for providing a fast sequential switching operation in the subnanosecond range, comprising: a. a block primarily composed of p-type semiconductor material having a cathode end and an anode end, said p-type material being capable of impact ionization in response to electron injection when maintained within a predetermined range of temperatures, b. means maintaining said block at one of said temperatures in said predetermined range, c. pulse generator means connected to said cathode end and said anode end for biasing said block with a voltage pulse having a magnitude sufficient To allow creation of an impact ionization wavefront upon electron injection, said impact ionization wavefront thereafter traveling from said cathode to said anode at a very high rate of speed, d. a plurality of electrodes spaced along the length of said block intermediate said cathode and said anode ends, and e. a plurality of detectors, each of said plurality of detectors being coupled to at least one of said plurality of electrodes and being adapted to be actuated upon passage of said impact ionization wavefront by its respective electrodes.
 2. An apparatus as recited in claim 1, wherein at least one of said detectors is responsive to resistance of said block between its corresponding electrodes.
 3. An apparatus as recited in claim 1, wherein at least one of said detectors is responsive to the potential of said block at its corresponding electrode.
 4. An apparatus as recited in claim 1, wherein said p-type material is indium antimonide.
 5. An apparatus as recited in claim 4, wherein the magnitude and polarity of said biasing voltage pulse is sufficient to create an impact ionization wavefront adjacent said cathode end.
 6. An apparatus as recited in claim 1, wherein the magnitude and polarity of said biasing voltage pulse is sufficient to create an impact ionization wavefront adjacent said cathode end.
 7. An apparatus as recited in claim 1, wherein said p-type material comprises mercury cadmium telluride.
 8. An apparatus as recited in claim 1, wherein said p-type material comprises lead tin telluride. 